Obesity and type 2 diabetes (T2D) are epidemics in the U.S. and abroad. A better understanding ofthe mechanisms that promote obesity and induce T2D is urgently needed to stem the tide of these epidemics and to control their cardiovascular complications. We suggest that endothelial dysfunction induced by nutrient excess is the primary cause that leads to metabolic changes resulting in an increase in adiposity and whole-body insulin resistance. Our studies show that overexpression of eNOS in mice prevents diet-induced obesity. These changes are accompanied by systemic and adipose tissue-specific changes in metabolism. Our metabolomic analyses indicate that eNOS over expression increases the abundance of circulating bile acids, which have been shown to be potent effectors of metabolism and regulators of adipose tissue phenotype. Nevertheless, we do not know how NO regulates bile acid production or which metabolic pathways triggered by bile acids are responsible for the lean phenotype of eNOS-TG mice. Therefore, we will test the hypothesis that NO exerts an anti-obesogenic effect by regulating bile acid metabolism, which promotes the development of adipocytes into a novel lean phenotype characterized by high mitochondrial content and fat burning capacity. To test this hypothesis, we will: (1) Examine the effects of NO on diet induced obesity; (2) Determine how NO regulates metabolism; and (3) Elucidate the mechanisms regulating adipocyte phenotype. The experimental approaches in these aims will test whether NO directly regulates obesity and whether the anti-obesity effects of NO are mediated through the ability of eNOS to increase bile acid production and to regulate adipose tissue phenotype. The results of these studies will develop a strong platform for constructing a competitive ROI application and will lead to a new understanding of the role of NO in regulating the metabolic changes that contribute to diabetes and obesity. These studies could lay the groundwork for the development of novel therapeutic interventions to prevent, manage or reverse obesity and insulin resistance.